Handover during carrier aggregation operation in wireless communication network

ABSTRACT

Methods for performing handovers and addition of carriers during carrier aggregation operation are described. A mobile station can indicate failure to perform downlink synchronization to some but not all cells of a target eNB, in response to a handover command. The mobile station can activate carriers based on various combinations of transmission of random access preambles, reception of random access response messages and transmission of handover complete messages. A base station can activate carriers based on various combinations of reception of random access preambles, transmission of random access response messages and reception of handover complete messages.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 13/052,266, filed on Mar. 21, 2011, which claimspriority to U.S. provisional Application No. 61/331,353 filed on May 4,2010, the disclosures of which are incorporated herein by reference.

FILED OF THE DISCLOSURE

The present disclosure relates generally to wireless communications, andmore specifically to Carrier Aggregation.

BACKGROUND

Carrier Aggregation will be used in 3GPP LTE networks to provideimproved data rates to users. Carrier aggregation consists oftransmitting data to or receiving data from the UE on multiple carrierfrequencies referred to herein as “component carriers” or “carriers”.The wider bandwidth enables higher data rates.

The present disclosure considers the problem of performing handover of amobile station from a source base station (also known as source node-B,source evolved-node-B, or source eNB) to a target base station (alsoknown as target node-B, target evolved-node-B or target eNB) whenCarrier aggregation (CA) is in use. The currently specified LTE handoverprocedure supports handover of a mobile station while operating on asingle carrier (i.e. in the absence of carrier aggregation).

For carrier aggregation (CA) operation, a mobile station (also referredto as user equipment or “UE”) can be configured with a set of componentcarrier (CCs). Component carriers can be downlink (DL) componentcarriers (used for transmission from an eNB to UEs) or uplink (UL)component carriers (used for transmission from UEs to eNBs). Typically,each uplink CC has a corresponding downlink CC to which it is paired.The pairing normally ensures that if the UE transmits a random accesspreamble on an uplink CC, the response message to the random accesspreamble transmission is received on the paired downlink CC. There canbe situations where a downlink CC is configured but the paired uplink CCis not configured, and vice versa. Some of the configured CCs may beactivated. The activated CCs can be used to send and receive data (i.e.,the activated CCs can be used for scheduling) to and from the UE. The UEhas up to date system information for all configured CCs or at least theconfigured CCs that the network expects to activate. Therefore, after aCC has been configured, it can be quickly activated, withoutexperiencing the delay due to delivery of relevant system informationneeded to perform communication on such a CC. Thus, when there is a needfor aggregating multiple CCs (e.g., a large burst of data), the networkcan activate configured CCs. The maintenance of a configured CC set inaddition to an activated CC set enables battery conservation in the UEby ensuring that CCs need to be activated only when there is asubstantial amount of data to be transmitted.

The currently specified LTE handover procedure requires the network tofirst bring the set of activated CCs down to a single CC before thehandover. The network can re-activate CCs as needed after the handoveris completed. The problem with such a handover procedure is that whencarrier aggregation is being used, it can cause significantinefficiencies and delays in the data transfer. As mentioned above,carrier aggregation is expected to be used (i.e., multiple CCs areexpected to be activated) only when there is a substantial amount ofdata to be transmitted). If a handover occurs during such a datatransfer, this option would result in buffering of large quantities ofdata at the source eNB and subsequent transfer of the data to the targeteNB (and possibly also buffering of large quantities of data at the UE).It can also cause excessive buffering of data at the target eNB asre-activation of CCs at the target eNB can take significant time afterthe handover.

Therefore it is beneficial to have handover mechanisms that are moresuitable for carrier aggregation operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a handover procedure that can be used in the absence ofcarrier aggregation.

FIG. 2 shows the component carrier coverage scenarios that a handoverprocedure needs to support in the presence of carrier aggregation.

FIG. 3 illustrates a method for handover during carrier aggregationoperation when multiple CCs are configured and activated upon handover.

FIG. 4 illustrates a method for handover during carrier aggregationoperation when multiple CCs are configured and activated upon handoverand the mobile station can indicate failure to activate some of the CCs.

FIG. 5 illustrates a method for handover during carrier aggregationoperation that reduces the interruption during a handover with carrieraggregation.

FIG. 6 illustrates a method for activation of additional CCs uponhandover using an activation message from the target eNB.

FIG. 7 illustrates a method for handover during carrier aggregationoperation when multiple timing advance configurations are necessarywherein multiple handover complete messages are sent.

FIG. 8 illustrates a method for handover during carrier aggregationoperation when multiple timing advance configurations are necessarywherein a single handover complete message is sent.

FIG. 9 illustrates a method for handover during carrier aggregationoperation when multiple timing advance configurations are necessarywherein a single handover complete message is sent.

DETAILED DESCRIPTION

A method for handover during carrier aggregation operation enables ahandover procedure which overcomes the deficiencies described above. Theembodiments are described in terms of a 3GPP Long term Evolution (LTE)system; however, it should be clear that the invention and theembodiments are equally applicable to other wireless communicationsystems. The scenarios which the handover procedure needs to address aresummarized below and also illustrated in FIG. 2:

Same component carriers used for carrier aggregation in both the sourceeNB and the target eNB;

At least one common component carrier used for carrier aggregation inboth the source eNB and the target eNB; and

No component carriers in common among components carriers used forcarrier aggregation in the source eNB and the target eNB.

In a Frequency division duplex (FDD) system, a downlink CC has acorresponding uplink CC. As in LTE Release 8 an accurate timing advanceis needed to transmit on an uplink CC. The timing advance value isprimarily a function of the propagation characteristics of thefrequency. A single timing advance is normally adequate when all uplinkCCs in use belong to the same frequency band. However, different timingadvance values for different uplink CCs may be needed in the followingsituations:

The uplink CCs belong to different frequency bands;

The network employs repeaters or remote radio heads; and

A base station uses different sectorization on different frequencies(for example, an eNB may use 3 sectors on CC1 and 6 sectors on CC2,leading to different propagation characteristics due to the differentantennae).

Some embodiments are directed at handover when only a single timingadvance is needed for all uplink CCs. Other embodiments are directed athandover when more than one timing advance is needed for the uplink CCs.

According to a first embodiment, illustrated in FIG. 3, the handovercommand may signal (a) the set of CCs to be configured by the UE uponhandover, and (b) the set of CCs to be (configured and) activated uponhandover. The handover command may also signal the CC on which the UE isrequired to transmit a RACH preamble (the RACH preamble may also besignaled). The handover command may be generated by the target eNB anddelivered to the UE through the source eNB. Alternatively, the handovercommand may be generated by the source eNB and directly delivered to theUE by the source eNB.

The handover complete serves as an indication that that the UE is readyto receive PDCCH on all of the CCs to be configured and activated.Alternatively, the handover complete can serve as an indication that theUE is downlink synchronized on all the CCs and subsequently the networkcan activate individual CCs. In a first step the UE can send ameasurement report. The measurement report can trigger an inter-eNBhandover. The target eNB can determine the CC set to be used uponhandover and the uplink CC on which the UE can transmit a random accesschannel preamble (RACH preamble). In order to enable the target eNB toselect CCs of good signal strength or signal quality, the source eNB canoptionally forward measurement information reported by the mobilestation. Alternatively, the source eNB can determine the CC set to beused upon handover and the uplink CC on which the UE can transmit a RACHpreamble. A handover command is transmitted to the UE that can indicatethe set of downlink CCs to be configured upon handover (CC set 1) andthe set of downlink CCs to be activated upon handover (CC set 2). Thehandover command can also indicate a specific random access preamblethat the UE is required to use as a part of the handover procedure.

The UE can transmit a random access preamble and receive a random accessresponse. The UE can then transmit a handover complete message. Thetransmission of the random access preamble and the handover completemessage can occur on different uplink CCs. If CC set 2 was included inthe handover command, upon transmitting the handover complete, the UEcan consider each of the CCs in CC set 2 to be activated. The UE canthen monitor each of the CCs in CC set 2 for a physical downlink controlchannel (PDCCH) or any other control channel. In order to be able tomonitor a CC for a control channel, the UE needs to first performdownlink synchronization to the cell from which the control channel isexpected. Alternatively, upon transmitting the handover completemessage, the UE can consider a single downlink CC to be activated (forexample, the CC on which the random access response was received). Thetarget eNB can then individually activate other CCs as needed.

In some cases, the UE may not be able to perform downlinksynchronization to one or more of the CCs in CC set 2 (for example, theUE may not be able to detect a synchronization channel on one or more ofthe CCs in CC set 2). According to a second embodiment illustrated inFIG. 4, if the UE is unable to perform downlink synchronization on theDL CC paired to the UL CC on which it is expected to perform the RACHtransmission, then the handover is considered to have failed and a radiolink recovery procedure can follow. If the UE is unable to performdownlink synchronization on a DL CC that is not paired to the UL CC onwhich it is expected to perform the RACH transmission, then the handovercomplete message can indicate the CCs on which the UE does not havedownlink synchronization. This allows for the case of “partial success”of the handover—the target eNB can continue communication with the UE onthe at least one CC that the UE has successfully DL synchronized to andtry to activate alternate CCs for aggregation.

According to another embodiment, the handover command can configuremultiple CCs of the target eNB (CC set 1) and designate multiple CCs tobe activated (CC set 2). One CC pair (DL CC & UL CC) of the target canbe designated for the random access procedure for handover, oralternatively, the UE can pick a CC pair for the random access procedurefor handover from the configured CCs. The UE can perform DLsynchronization on all the configured DL CCs (CC set 1) and can performUL sync on one of the configured UL CC. At handover completion themultiple CCs designated for activation (CC set 2) on the target eNB areconsidered activated. Consequently, the UE can monitor each of the CCsin CC set 2 for a physical downlink control channel (PDCCH) or any othercontrol channel. If the UE is not able to perform DL synchronization toone or more of multiple CCs configured or designated for activation,then a subset of the CCs is activated (the ones on which the UE was ableto perform DL synchronization) and the UE can indicate the failure toperform DL synchronization on some CCs to the network (for example, inthe handover complete message). Upon completion of the handover thetarget eNB can activate additional DL CCs from the configured set.

According to another embodiment illustrated in FIG. 5, the UE candetermine whether the CC on which the UE is required to perform randomaccess preamble transmission to the target eNB for handover is anactivated CC at the source eNB. If the CC on which the UE is required toperform random access preamble transmission is an activated CC, then theUE can stop looking for control channels on that DL CC and thecorresponding UL CC is used for the random access preamble transmission.Meanwhile, DL data can be received from source eNB on the other DL CCsand UL transmissions can be continued on corresponding UL CCs. The UEcan receive a timing advance (TA) command from the target eNB (forexample, in the random access response message). Upon receiving the TAcommand, the UE can stop monitoring for control channels on the sourceeNB CCs and start monitoring for control channels on the target eNB CCs.Such a mechanism can substantially reduce interruption during thehandover.

According to another embodiment illustrated in FIG. 6, the handovercommand can configure multiple CCs (CC set 1) on the target eNB, but asingle CC can be designated to be activated regardless of the number ofCCs activated at the source eNB. The CC designated to be activated canbe referred to as the Primary CC. The UE performs DL synchronization tothe CCs in CC set 1. The UE can perform random access preambletransmission on the UL CC corresponding to the CC designated to beactivated and receives a random access response. The UE then transmits ahandover complete message. After successful handover the target eNB canactivate additional CCs from the configured set. Being synchronized tothe multiple DL CCs of the target eNB ensures that the target eNB canquickly activate them after handover.

In order to activate additional CCs after handover, the target eNB canuse an activation message, such as a MAC (medium access control) messageindicating CC activation. Such a message will be referred to as anactivation message or a MAC activation message. The activation messagecan be sent by the target eNB, with the random access response message,if the target eNB can determine the identity of the UE based on therandom access preamble that is received. The target eNB can determinethe identity of the UE based on the random access preamble, if apre-assigned dedicated preamble is used for the random access during thehandover. The UE can then perform activation of the CCs indicated in theactivation message and then transmit a handover complete message. Upontransmission of the handover complete message, the UE can monitor theactivated CCs for control channels. However, the UE may not be able tosuccessfully activate all the CCs indicated in the activation message.This may be due to (a) there not being enough time from the reception ofthe activation message to the transmission of a response to theactivation message (such as the handover complete) for the UE to performdownlink synchronization to the cells of the target eNB on the CCsindicated in the activation message, or (b) UE not having performedadequate radio resource management (RRM) measurements on the CCsdesignated for activation. If the UE is unable to activate one or moreCCs indicated in the activation message, it can indicate the failure toactivate in the handover complete message.

According to another embodiment, in order to activate additional CCsafter handover, the target eNB can use an activation message. Theactivation message can be sent by the target eNB with the random accessresponse message, if the target eNB can determine the identity of the UEbased on the random access preamble that is received. The UE can thenperform activation of the CCs indicated in the activation message andthen transmit a handover complete message. Upon transmission of thehandover complete message, the UE can monitor the activated CCs forcontrol channels. However, the UE may not be able to successfullyactivate all the CCs indicated in the activation message. This may bedue to (a) there not being enough time from the reception of theactivation message to the transmission of a response to the activationmessage (such as the handover complete) for the UE to perform downlinksynchronization to the cells of the target eNB on the CCs indicated inthe activation message, or (b) UE not having performed adequate radioresource management (RRM) measurements on the CCs designated foractivation. In order to reduce the likelihood of activation failure, theUE can maintain additional radio frequency chains activated uponreceiving the handover command and use the additional radio frequencychains to perform various activities. Such activities can includeperforming downlink synchronization to the cells of the target eNB on atleast some of the CCs designated for activation and performingmeasurements on at least some of the CCs designated for activation.Maintaining additional radio frequency chains activated can includeretuning or reconfiguring some radio frequency chains that were beingused for communication with the source eNB, or activating radiofrequency chains that were not in use when the handover command wasreceived.

According to another embodiment directed at activation of CCs in theabsence of a handover, a UE can receive an activation message orderingthe activation of multiple CCs. The UE may be unable to activate one ormore of the indicated CCs due to, for example, the UE not havingperformed adequate radio resource management (RRM) measurements on theCCs designated for activation. The UE can activate the CCs that it isable to successfully activate. The UE can then indicate to the networkthe CCs that it is unable to activate.

Further embodiments are described below that are directed at carrieraggregation when multiple timing advance values are needed. For example,if the uplink CCs to be used for carrier aggregation are from differentfrequency bands, different timing advance values may be needed for theuplink CCs in the different bands. Other situations where there may be aneed for different timing advance values include situations where thenetwork uses repeaters or remote radio heads, and situations where thenetwork uses different sectorization on different bands. In suchscenarios, the UE may need to perform multiple random access procedures(on different uplink CCs) to obtain the independent timing advancevalues.

According to another embodiment illustrated in FIG. 7, the UE sends ameasurement report that triggers an inter-eNB handover. The target eNBcan determine the CC set to be used by the UE at the target eNB and candetermine the uplink CCs on which the UE is required to transmit randomaccess preamble (RACH-CC set). The Network can indicate the sets ofdownlink CCs to be configured (CC set 1) and to be configured andactivated (CC set 2) in the handover command. The network can alsoprovide random access preambles to use on each of the CCs in the RACH-CCset. Alternatively, the source eNB can determine he CC set to be used bythe UE at the target eNB and can determine the uplink CCs on which theUE is required to transmit random access preamble (RACH-CC set), and canalso provide the random access preambles to use on each of the CCs inthe RACH-CC set. The following types of parings are possible between CCset 2 and the RACH-CC set: (a) one downlink CC is paired to one uplinkCC in CC set 2, (b) Multiple downlink CCs in CC set 2 are paired to oneCC in the RACH-CC set, and (c) Multiple uplink CCs in RACH-CC set arepaired to one downlink CC in CC set 2.

The UE can transmit random access preambles on each of the CCs in theRACH-CC set and in response to the random access preamble, receive arandom access response message corresponding to the random accesspreamble transmission. The random access response can include a timingadvance value for the corresponding uplink CC.

The UE can then transmit multiple handover complete messages. There canbe a correspondence between the handover complete messages and the DLCCs in CC set 2. For example, a handover complete can use an uplinkgrant signaled in the random access response message. Thus the handovercomplete message can correspond to all the DL CCs that are paired to theUL CC on which the random access preamble was transmitted. Upontransmitting a handover complete message, the UE can consider all the DLCCs mapped to the handover complete message to be activated. As aconsequence, the UE can monitor the downlink CCs corresponding to thehandover complete message for control channels. Correspondingly, thetarget eNB can delay transmitting of control channels to the UE on adownlink CC until it has received the mapped handover complete message.

According to another embodiment illustrated in FIG. 8, the UE sends ameasurement report that triggers an inter-eNB handover. The target eNBor the source eNB can determine the CC set to be used by the UE at thetarget eNB and can determine the uplink CCs on which the UE is requiredto transmit random access preamble (RACH-CC set). The Network canindicate the sets of downlink CCs to be configured (CC set 1) and to beconfigured and activated (CC set 2) in the handover command and can alsoprovide random access preambles to use on each of the CCs in the RACH-CCset.

The UE can transmit random access preambles on each of the CCs in theRACH-CC set and in response to the random access preamble, receive arandom access response message corresponding to the random accesspreamble transmission. The random access response can include a timingadvance value for the corresponding uplink CC.

The UE transmits a single handover complete message. The single handovercomplete message can be transmitted when the last outstanding randomaccess response is received by the UE. The handover complete message canbe transmitted using an uplink grant in the last random access responsemessage. Upon transmitting the handover complete, the UE can considerall the CCs in CC set 2 to be activated. Consequently, it can monitorall the CCs in CC set 2 for control channels. Correspondingly, thetarget eNB can delay transmission of control channels to the UE on anyof the CCs in CC set 2 until it has received the handover completemessage.

According to another embodiment illustrated in FIG. 9, the UE can send ameasurement report that triggers an inter-eNB handover. The target eNBor the source eNB can determine the CC set to be used by the UE at thetarget eNB and can determine the uplink CCs on which the UE is requiredto transmit random access preamble (RACH-CC set). The Network canindicate the sets of downlink CCs to be configured (CC set 1) and to beconfigured and activated (CC set 2) in the handover command and can alsoprovide random access preambles to use on each of the CCs in the RACH-CCset.

The UE can transmit random access preambles on each of the CCs in theRACH-CC set and in response to the random access preamble, receive arandom access response message corresponding to the random accesspreamble transmission. The random access response can include a timingadvance value for the corresponding uplink CC.

The UE transmits a single handover complete message. The single handovercomplete message can be transmitted when one of the random accessresponse messages is received. The handover complete message can betransmitted using an uplink grant in the random access response message.Consequently, the UE monitors a DL CC for control channels if the timingadvance for the paired uplink CC (or CCs) has been obtained and thecorresponding handover complete has been transmitted. Correspondingly,the target eNB does can delay transmission of control channels to the UEon a set of downlink CCs until it has received the designated randomaccess preamble on the corresponding uplink CC and also received thecorresponding handover complete. Alternatively, the target eNB can delaythe transmission of control channels to the UE on a set of downlink CCsuntil it has sent a random access response in response to a randomaccess preamble received on the corresponding uplink CCs, and alsoreceived the corresponding handover complete.

In the foregoing specification, specific embodiments have beendescribed. However, one of ordinary skill in the art appreciates thatvarious modifications and changes can be made without departing from thescope of the invention as set forth in the claims below. Accordingly,the specification and figures are to be regarded in an illustrativerather than a restrictive sense, and all such modifications are intendedto be included within the scope of present teachings.

The benefits, advantages, solutions to problems, and any element(s) thatmay cause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeatures or elements of any or all the claims. The invention is definedsolely by the appended claims including any amendments made during thependency of this application and all equivalents of those claims asissued.

It will be appreciated that some embodiments may utilize one or moregeneric or specialized processors (or “processing devices”) such asmicroprocessors, digital signal processors, customized processors andfield programmable gate arrays (FPGAs) and unique stored programinstructions (including both software and firmware) that control the oneor more processors to implement, in conjunction with certainnon-processor circuits, some, most, or all of the functions of themethod and/or apparatus described herein. Alternatively, some or allfunctions could be implemented by a state machine that has no storedprogram instructions, or in one or more application specific integratedcircuits (ASICs), in which each function or some combinations of certainof the functions are implemented as custom logic. Of course, acombination of the two approaches could be used.

Moreover, an embodiment can be implemented as a computer-readablestorage medium having computer readable code stored thereon forprogramming a computer (e.g., comprising a processor) to perform amethod as described and claimed herein. Examples of suchcomputer-readable storage mediums include, but are not limited to, ahard disk, a CD-ROM, an optical storage device, a magnetic storagedevice, a ROM (Read Only Memory), a PROM (Programmable Read OnlyMemory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM(Electrically Erasable Programmable Read Only Memory) and a Flashmemory. Further, it is expected that one of ordinary skill,notwithstanding possibly significant effort and many design choicesmotivated by, for example, available time, current technology, andeconomic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of generating such softwareinstructions and programs and ICs with minimal experimentation.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed embodiment. Thus the following claims arehereby incorporated into the Detailed Description, with each claimstanding on its own as a separately claimed subject matter.

1. (canceled)
 2. A method in a target base station for handover of amobile station from a source base station to the target base station,the target base station supporting a plurality of cells, each cell ofthe plurality of cells operating on a different component carrierfrequency, the method comprising: generating a handover commandindicating a first set of cells of the target base station to beconfigured for communication, wherein the first set of cells comprisesmore than one cell; receiving a message indicating a partial completionof handover, the message being received after the mobile stationperforms downlink synchronization to at least one but not all of thefirst set of cells; transmitting an activation message indicating asecond set of cells of the target base station to be activated; andmonitoring, after the mobile station performs downlink synchronizationto at least one of the second set of cells, for control transmissionsfrom the cells in the second set of cells to which the mobile stationperformed downlink synchronization.
 3. The method according to claim 2wherein the message indicating a partial completion of handover includesa message identifying at least one of the cells of the target basestation to which the mobile station failed to perform downlinksynchronization.
 4. The method according to claim 2 wherein the messageindicating a partial completion of handover includes a messageidentifying a subset of cells of the target base station, the subset ofcells being a subset of the first set of cells, and the subset of cellsnot including all of the first set of cells.
 5. The method according toclaim 2 wherein the first set of cells comprises the set of cells of thetarget base station operating on carrier frequencies to be configured bythe mobile station.
 6. The method according to claim 2 wherein the firstset of cells comprises the set of cells of the target base stationoperating on carrier frequencies to be activated by the mobile station.